A rotary valve provided with a tubular housing contained in a piston rod of a damper, and a tubular valve body rotatably inserted in this housing is known (See Japanese Utility Model Publication No. 62-117342). In this rotary valve, a port is formed by an oval hole extending along the circumferential direction and with variable widths in the axial direction at the side part of the housing, and the tubular valve body is provided with a circular hole capable of opposing to a part of the port.
In the conventional rotary valve, the tubular valve body is rotated in relation to the housing, thereby changing the flow channel area of the hydraulic oil by changing the degree of overlapping between the port and the circular hole. In another word, the damping force generated by the damper is adjusted by changing the pressure loss caused when the hydraulic oil is passing through the rotary valve.